Title

Author

Degree Type

Dissertation

Date of Award

2009

Degree Name

Doctor of Philosophy

Department

Anthropology

Major

Ecology and Evolutionary Biology

First Advisor

Jill Pruetz

Abstract

The study of orangutan (Pongo spp.) cognitive complexity regarding tool use and manufacture is essential for accurate modeling of hominid evolution. Orangutans demonstrated comprehension of causal relations in four studies of tool use and manufacture. Study subjects were 3 captive orangutans (Great Ape Trust, Des Moines, IA). The distinction between rigid and flimsy tools was explored in the first study. The apes demonstrated comprehension by choosing the rigid tools significantly more than the flimsy tools to solve a problem. In the second study, the results provide support for orangutan comprehension of tool and apparatus properties in an extension of the tube task (Visalberghi & Trinca, 1989). In the first of two experiments, orangutans were required to manipulate or modify tools for retrieval of a reward from a tube. The apes demonstrated a greater degree of comprehension than has been reported previously for this problem (Visalberghi, Fragaszy, & Savage-Rumbaugh, 1995). A second variant of the tube task, the blocked tube, required sequential modification to the tube before tool insertion. Although the results of the second experiment were not entirely conclusive, comprehension of sequential modifications is suggested. The trap-tube apparatus, involving horizontal and vertical tubes containing obstacles, was presented in the third study. The results of the trap tube studies support the conclusion that causal cognition is a capacity present in orangutans, with individual variation an important factor in its detection. In the final study, tool manufacture was explored by presenting the apes with a series of novel experiments requiring construction of tools allowing for absorption, stabbing, raking and probing. Tool modifications were recorded in all experiments, with differences between tools for a given experiment supporting an orangutan "tool kit."

The results of these studies of tool use and manufacture by orangutans support the interpretation that these apes use tools with cognitive complexity involving an understanding of causal relations. Suggestions that the understanding of causal relations is a uniquely human ability and that the nonhuman great apes may lack this ability are not supported. Based on the accumulated data of the four studies presented in this dissertation, the hypothesis was supported that orangutans comprehend relevant tool properties and have or can develop an understanding of causal relations. These results support that the last common ancestor shared by the lineage leading to the African and Asian great apes, including humans, had the ability for causal understanding.